Method of heat treating an article

ABSTRACT

The invention describes a method of heat-treating an article, which includes a first step of heating the article to a temperature of 400° C. to 500° C. at a pressure of 1 to 3 millibar in an atmosphere comprising hydrogen for a period of 0.1 to 50 hours to produce a hot article, a second step of heating the hot article at a temperature of 400° C. to 500° C. at a pressure of 1 to 3 millibar in an atmosphere comprising at least one of hydrogen, argon, and nitrogen, for 0.1 to 50 hours to produce a preliminary heat treated article, and a third step of heating the preliminary heat treated article at a temperature of 400° C. to 500° C. at a pressure of 1 to 3 millibar in an atmosphere comprising at least one of hydrogen, nitrogen, and a hydrocarbon gas, for 0.1 to 50 hours; to produce a heat-treated article.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method of heat treating an article.More specifically, the invention relates to a method of heat treatingstainless steel articles such as bite type of fittings.

2. Discussion of the Related Art

Various methods of heat treating stainless steel articles are known inthe art. U.S. Pat. No. 6,238,490 refers to a process for heat-treatingaustenitic steel articles to produce a hardened surface by heating thearticle in a gas mixture of hydrogen and methane. US patent applicationnumber 20120018052 describes a process for carburization of an articlemade of steel, in a vacuum furnace in the presence of a hydrocarboncarburizing gas. While, Chinese patent application number 105603359seems to describe a method for glow ion carbonization of stainless steelfor enhanced surface hardness and corrosion resistance. However, thesemethods often need special heat treatment furnaces.

Therefore, there is a need in the art for a process for heat treating astainless steel article at low temperatures to impart a balance of goodsurface hardness and good corrosion resistance which can be processed incommonly available furnaces.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a method of heat treating anarticle. The method includes a first step of heating the article to atemperature in a range of about 400° C. to about 500° C. at a pressurein a range of about 1 millibar to about 3 millibar in an atmospherecomprising hydrogen, for a period of about 0.1 hours to about 50 hoursto produce a hot article. The method includes a second step of heatingthe hot article at a temperature in a range of about 400° C. to about500° C. at a pressure within a range of about 1 millibar to about 3millibar in an atmosphere comprising at least one of hydrogen, argon,and nitrogen, for a period of about 0.1 hours to about 50 hours toproduce a preliminary heat treated article. Further, the method includesa third step of heating the preliminary heat treated article at atemperature in a range of about 400° C. to about 500° C. at a pressurein a range of about 1 millibar to about 3 millibar in an atmospherecomprising at least one of hydrogen, nitrogen, and a hydrocarbon gas,for a period of about 0.1 hours to about 50 hours; to produce aheat-treated article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a method of heat treating an article in accordance to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the specification and the claims which follow, reference will be madeto a number of terms which shall be defined to have the followingmeanings:

The singular forms “a”, “an” and “the” include plural referents unlessthe context clearly dictates otherwise. “Optional” or “optionally” meansthat the subsequently described event or circumstance may or may notoccur, and that the description includes instances where the eventoccurs and instances where it does not. “Substantially” means a range ofvalues that is known in the art to refer to a range of values that areclose to, but not necessarily equal to a certain value.

Other than in the examples or where otherwise indicated, all numbers orexpressions referring to quantities of ingredients, reaction conditions,and the like, used in the specification and claims are to be understoodas modified in all instances by the term “about.”

As used herein, the term “substantially” and its variations are definedas being largely but not necessarily wholly what is specified asunderstood by one of ordinary skill in the art.

Various numerical ranges are disclosed herein. Because these ranges arecontinuous, they include every value between the minimum and maximumvalues. The endpoints of all ranges reciting the same characteristic orcomponent are independently combinable and inclusive of the recitedendpoint. Unless expressly indicated otherwise, the various numericalranges specified in this application are approximations. The endpointsof all ranges directed to the same component or property are inclusiveof the endpoint and independently combinable.

As used herein, “combinations thereof” is inclusive of one or more ofthe recited elements, optionally together with a like element notrecited, e.g., inclusive of a combination of one or more of the namedcomponents, optionally with one or more other components notspecifically named that have essentially the same function. As usedherein, the term “combination” is inclusive of blends, mixtures, alloys,reaction products, and the like.

As used herein, the term “bite type fitting” refers to an article thatis used for joining two tubes. The bite type fitting is composed of anouter compression nut and an inner compression ferrule. When the nut istightened, the ferrule is compressed between the nut and the body of thefitting, thus forming a tight, leak-proof joint.

As used herein the term “standard cubic centimetres per minute” or“sccm” is defined as a gas flow rate corresponding to a cubic centimeterof gas flowing in one minute.

One embodiment of the present invention is a method of heat treating anarticle, the method includes a first step of heating the article to atemperature in a range of about 400° C. to about 500° C. at a pressurein a range of about 1 millibar to about 3 millibar in an atmospherecomprising hydrogen, for a period of about 0.1 hours to about 50 hoursto produce a hot article, a second step of heating the hot article at atemperature in a range of about 400° C. to about 500° C. at a pressurewithin a range of about 1 millibar to about 3 millibar in an atmospherecomprising at least one of hydrogen, argon, and nitrogen, for a periodof about 0.1 hours to about 50 hours to produce a preliminary heattreated article. The method further includes a third step of heating thepreliminary heat treated article at a temperature in a range of about400° C. to about 500° C. at a pressure in a range of about 1 millibar toabout 3 millibar in an atmosphere comprising at least one of hydrogen,nitrogen, and a hydrocarbon gas, for a period of about 0.1 hours toabout 50 hours; to produce a heat-treated article.

As depicted in FIG. 1 according to an embodiment of the presentinvention, the method 100 consists of the following steps: heating 102the article to a temperature in a range of about 400° C. to about 500°C. at a pressure in a range of about 1 millibar to about 3 millibar inan atmosphere comprising hydrogen, for a time of about 0.1 hours toabout 50 hours to produce a hot article; heating 104 the hot article ata temperature in a range of about 400° C. to about 500° C. at a pressurewithin a range of about 1 millibar to about 3 millibar in an atmospherecomprising at least one of hydrogen, argon, and nitrogen, for a time ofabout 0.1 hours to about 50 hours to produce a preliminary heat treatedarticle, and heating 106 the preliminary heat treated article at atemperature in a range of about 400° C. to about 500° C. at a pressurein a range of about 1 millibar to about 3 millibar in an atmospherecomprising at least one of hydrogen, nitrogen, and a hydrocarbon gas,for a time of about 0.1 hours to about 50 hours to produce aheat-treated article.

In an embodiment of the present invention, the article is subjected to acleaning step before being heat treated. The cleaning step may includebut not limited to a water washing step, an acetone washing step, adegreasing step, a chemical cleaning step, a solvent cleaning step, anultrasonic cleaning step, a plasma cleaning step, an electrochemicalcleaning step, or any other such cleaning step commonly known to oneskilled in the art. In an embodiment of the present invention, thearticle is subjected to an acetone washing step.

In an embodiment of the present invention, the temperature in the firststep and the second step may be within a range of about 380° C. to about440° C. In another embodiment of the present invention, the temperaturein the first step and the second step is 425° C. In an embodiment of thepresent invention, the temperature in the third step may be within arange of about 380° C. to about 440° C.

In an embodiment of the present invention, the pressure in the firststep may be within a range of about 1 millibar to about 3 millibar. Inan embodiment of the present invention, the pressure in the first stepmay be about 2 millibar. In an embodiment of the present invention, thepressure in the second step may be within a range of about 3 millibar toabout 5 millibar. In an embodiment of the present invention, thepressure in the second step may be 4 millibar. In an embodiment of thepresent invention, the pressure in the third step may be within a rangeof about 1 millibar to about 3 millibar. In an embodiment of the presentinvention, the pressure in the third step may be about 2 millibar.

The method further includes a third step of heating the preliminary heattreated article in an atmosphere comprising at least one of hydrogen,nitrogen, and a hydrocarbon gas, for a period of about 0.1 hours toabout 50 hours; to produce a heat-treated article. In an embodiment ofthe present invention, the hydrocarbon may be methane, ethane, propane,butane, or a combination thereof. In an example embodiment of thepresent invention, the hydrocarbon gas may be methane.

In an embodiment of the present invention, the first step may be carriedout in a gas flow of about 200 sccm to about 600 sccm of hydrogen. Inanother embodiment of the present invention, the first step may becarried out in a gas flow of about 450 sccm of hydrogen. In anembodiment of the present invention, the second step may be carried outin a gas flow of about 400 sccm to about 1000 sccm of hydrogen, about200 sccm to about 800 sccm of nitrogen and about 10 sccm to about 50sccm of argon. In another embodiment of the present invention, thesecond step may be carried out in a gas flow of about 750 sccm ofhydrogen, about 500 sccm of nitrogen and about 10 sccm to about 50 sccmof argon. In yet another embodiment of the present invention, the thirdstep may be carried out in a gas flow of about 400 sccm to about 1000sccm of hydrogen, about 200 sccm to about 800 sccm of nitrogen, andabout 20 to about 100 sccm of a hydrocarbon. In an example embodiment ofthe present invention, the third step may be carried out in a gas flowof about 850 sccm of hydrogen, about 600 sccm of nitrogen, and about 50sccm of a hydrocarbon.

In an embodiment of the present invention, the first step may be carriedout in a period of about 0.1 hours to about 50 hours. In anotherembodiment of the present invention, the first step may be carried outin a period of about 6 hours. In an embodiment of the present invention,the second step may be carried out for a period of about 0.1 hours toabout 50 hours. In yet another embodiment of the present invention, thesecond step may be carried out for a period of about 12 hours. In anembodiment of the present invention, the third step may be carried outfor a period of about 0.1 hours to about 50 hours. In another oneembodiment of the present invention, the third step may be carried outfor a period of about 16 hours.

In an embodiment of the present invention, the method of heat treatingthe article further includes a step of tempering the heat treatedarticle. In an embodiment of the present invention, the method of heattreating the article further includes a step of tempering the heattreated article at a temperature of about 600° C., for a time of about4.5 hours. In another embodiment of the present invention, the method ofheat treating the article further includes a step of tempering the heattreated article at a temperature within a range of about 400° C. toabout 800° C., for a time of about 0.1 hours to about 50 hours. In yetanother embodiment of the present invention, the method of heat treatingthe article further includes a step of tempering the heat treatedarticle at a temperature within a range of about 550° C. to about 620°C., for a time of about 3.5 hours to about 6 hours.

In an embodiment of the present invention, the heat treated articleafter the tempering step has been carried out, has a Vickers hardnessfrom about 800 units to about 880 units. In an embodiment of the presentinvention, the heat treated article has a Vickers hardness from about650 units to about 800 units.

In an embodiment of the present invention, the article may be anaustenitic stainless steel article. In an embodiment of the presentinvention, the article may be composed of “SS 316” grade stainless steelor “SS304” grade stainless steel or “SS316Ti grade stainless steel.

In an embodiment of the present invention, the article may be a singleferrule fitting. In another embodiment of the present invention, thearticle may be a gear, a sprocket, a screw, a ball bearing, a rollerbearing, a piston pin, a firearm, a chain, a lock shackle, a watch case,a cam shaft, a crankshaft, and the like.

EXAMPLES

Example 1: Commercially available ferrules, such as those manufacturedby Fluid Controls Pvt. Ltd., Pune, India, were obtained for heattreatment. The ferrule to be heat treated was placed in a furnace andheated to 425° C. at a pressure of about 2 millibar in an atmospherecomprising a flow of about 400 sccm of hydrogen. The ferrule was held at425° C. for 2 hours in an atmosphere of 450 sccm of hydrogen, 30 sccm ofargon and 600 sccm of nitrogen, at a pressure of 3-5 mbar, for sixhours, followed by a step of heating the ferrule at 425° C. in anatmosphere of 700 sccm of nitrogen, 30 sccm of argon and 50 sccm ofmethane, for 4-6 hours. The ferrule was subsequently allowed to cool toroom temperature.

Example 2: A process similar to the process of example 1 was followed toheat treat a ferrule. In the current example 2, the heat treated ferrulewas subjected to a tempering step. A ferrule to be teat treated wasplaced in a furnace and heated to 425° C. at a pressure of about 2millibar in an atmosphere comprising a flow of about 400 sccm ofhydrogen. The ferrule was held at 425° C. for 2 hours in an atmosphereof 700 sccm of hydrogen, 30 sccm of argon and 650 sccm of nitrogen, at apressure of 3-5 mbar, for 6 hours, followed by a step of heating theferrule at 425° C. in an atmosphere of 700 sccm of nitrogen, 40 sccm ofargon and 60 sccm of methane, for 8-10 hours. The ferrule wassubsequently allowed to cool to room temperature. After the ferrulecooled to room temperature, the ferrule was tempered at a temperature of600° C. in vacuum for about 5 hours.

Heat treated ferrules of various diameters ranging from 6 mm to about 42mm outer diameter were tested for leaks according to ISO 19879 standard.Typically, the ferrule was fitted on to a stainless steel tube andtested at 6.3 MPa for a duration of at least three minutes. No leakswere detected in the heat treated ferrules. The heated treated ferruleswere tested for leakages due to shock and vibration according to BE EN61373(2010) standard. The heat treated ferrules were found to meet thespecifications laid down in BE EN 61373(2010) standard. The heat treatedferrules fitted on to stainless steel tubes were also tested for leaksdue to misalignment. The ferrules were fitted on to steel tubes andclamped. The tubes were then misaligned from the clamped position up to45 mm, and the ferrule was crimped. The heat treated ferrules wereobserved to pass the leak tests as per ISO 19879 standard, in spite ofthe misalignment.

Comparative Examples: Non-heat-treated ferrules, such as thosemanufactured by Fluid Controls Pvt. Ltd., Pune, India, were obtained forcomparison. The non-heat treated ferrules were subjected to the sametests as for the heat-treated ferrules, and the results were compared.

Non-heat treated ferrules were tested for leaks according to ISO 19879standard. The non-heat treated ferrules employed above were variousdiameters ranging from 6 mm to about 42 mm outer diameter as mentionedabove in the case of the heat treated ferrules. Typically, the ferrulewas fitted on to a stainless steel tube and tested at 6.3 MPa for aduration of at least three minutes. No leaks were detected in thenon-heat treated ferrules. The non-heat treated ferrules fitted on tostainless steel tubes were also tested for leaks due to misalignment.The non-heat treated ferrules were fitted on to steel tubes and clamped.The tubes were then misaligned from the clamped position up to 45 mm,and the ferrule was crimped. The non-heat treated ferrules were observedto fail the leak tests as per ISO 19879 standard. The leak tests onmisaligned fittings have not been seen to be reported previously inliterature.

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions may occur to those skilled in the art withoutdeparting from the invention. It should be understood that variousalternatives to the embodiments of the invention described herein may beemployed.

What is claimed is:
 1. A method of heat-treating an article, comprising:a first step of heating the article to a temperature in a range of about400° C. to about 500° C. at a pressure in a range of about 1 millibar toabout 3 millibar in an atmosphere comprising hydrogen, for a period ofabout 0.1 hours to about 50 hours to produce a hot article; a secondstep of heating the hot article at a temperature in a range of about400° C. to about 500° C. at a pressure within a range of about 1millibar to about 3 millibar in an atmosphere comprising at least one ofhydrogen, argon, and nitrogen, for a period of about 0.1 hours to about50 hours to produce a preliminary heat treated article; a third step ofheating the preliminary heat treated article at a temperature in a rangeof about 400° C. to about 500° C. at a pressure in a range of about 1millibar to about 3 millibar in an atmosphere comprising at least one ofhydrogen, nitrogen, and a hydrocarbon gas, for a period of about 0.1hours to about 50 hours; to produce a heat-treated article.
 2. Themethod of claim 1, wherein the article is a bite type fitting.
 3. Themethod of claim 1, wherein the article is an austenitic stainless steelarticle.
 4. The method of claim 1, wherein the temperature in the firststep and the second step is 425° C.
 5. The method of claim 1, whereinthe temperature in the third step is 425° C.
 6. The method of claim 1,wherein the hydrocarbon gas is methane.
 7. The method of claim 1,wherein the first step is carried out in a gas flow of about 200 sccm toabout 600 sccm of hydrogen.
 8. The method of claim 1, wherein the secondstep is carried out in a gas flow of about 400 sccm to about 1000 sccmof hydrogen; about 200 sccm to about 800 sccm of nitrogen; and about 10sccm to about 50 sccm of argon.
 9. The method of claim 1, wherein thethird step is carried out in a gas flow of about 400 sccm to about 1000sccm of hydrogen; about 200 sccm to about 800 sccm of nitrogen; andabout 20 sccm to about 100 sccm of a hydrocarbon.
 10. The method ofclaim 1, further comprising a step of tempering the article at atemperature within a range of about 400° C. to about 800° C., for a timeof about 0.1 hours to about 50 hours.